Japanese Patent Application Laid Open No. 62(1987)-55449 discloses a rotary throttle carburetor with a needle inserted into a metering tip portion of a fuel supply pipe to meter fuel by adjusting the outlet area of a fuel jet contained within the metering tip portion of the fuel supply pipe. Particularly, the needle is incorporated into the fuel supply pipe in a coaxial manner, whereby a fine amount of fuel can be controlled in the idle operation of a small engine.
Recently, exhaust gas emissions control standards have been applied to small engines, and as a result, a more stabilized supply of fuel is needed in these small engines. The fuel supply pipe in the conventional rotary throttle valve type carburetor has been formed from a metal pipe with a slit-like fuel jet machined therein. However, having to machine the slit limits the shape of the fuel jet and creates a burr which is difficult to remove. Further, it is difficult to adapt the jet to the various amounts of fuel required by different engines.
On the other hand, when the fuel supply pipe is molded of synthetic resin, the fuel jet can be simultaneously molded. It is possible to adapt to the amount of fuel required by various engines from an idle position to a full open position of the throttle valve. In the fuel supply line formed of synthetic resin it is necessary to make the wall of the proximal end press-fit portion thicker than that formed of metal to ensure sufficient strength to allow the supply line to be press-fit into a mounting hole of the carburetor body. However, when the wall thickness is increased, the dimensional stability is decreased. Further, the metering tip portion, which contains the fuel jet, needs to have increased wall thickness, beyond that required for adequate strength, to be balanced for molding with respect to the proximal end press-fit portion. This increased wall thickness is needed because if there is a large variance in the wall thickness within the fuel supply line, the amount of deformation at the time of molding the resin increases, further impairing the dimensional stability.
Further, when the fuel supply pipe is press-fitted into the carburetor body the decreased dimensional stability can cause a non-concentric fit between the fuel supply pipe and the mounting hole of the carburetor body. This eccentricity can further cause a non-concentric, and hence, a poor fit and even bending between the metering tip portion of the fuel supply pipe and the needle inserted therein.